1. Field of the Invention
The present invention relates to an image heating apparatus for heating a toner image formed on a material to be heated by applying an appropriate image forming process, such as an electrophotographic process, an electrostatic recording process, a magnetic recording process or the like, to an image bearing member, such as an electrophotographic photosensitive member, an electrostatic recording dielectric member, a magnetic recording magnetic member or the like.
2. Description of the Related Art
A film type fixing apparatus having a heater, a flexible sleeve that moves in contact with the heater, and a pressure roller (i.e., elastic roller) that forms a fixing nip in cooperation with the heater via the flexible sleeve is known as an image heating apparatus mounted in an electrophotographic type copying machine or printer.
This film type fixing apparatus heat-fixes an unfixed toner image onto a recording material by transferring and heating the recording material to be heated, which bears the unfixed toner image and is nipped at a fixing nip, as discussed in, e.g., Japanese Patent Application Laid-Open Nos. 2006-171630, 2001-100556, and 2003-122147.
The time required for the fixing apparatus to reach a fixable temperature is comparatively short since energization of the heater is started. Accordingly, a first printout time (FPOT) by a printer with this fixing apparatus to output a first image after input of a print command is short. Thus, this printer has an advantage in low power consumption during a waiting time for a print command.
FIGS. 14 and 15 illustrate a conventional film-type fixing apparatus. FIGS. 14 and 15 are respectively a perspective view and a side view each illustrating a pressure unit for pressurizing a heater against a pressure roller to form a fixing nip. FIG. 14 illustrates one longitudinal end side of the fixing apparatus. The one longitudinal end side and the other longitudinal end side of the fixing apparatus are configured to respectively have shapes that are substantially symmetrical with respect to a plane perpendicular to the longitudinal direction thereof. Thus, drawing of the other longitudinal end side thereof is omitted. The “longitudinal direction” thereof is defined as a direction perpendicular to a recording material conveying direction in a recording material surface.
This fixing apparatus has a heating unit 126, a pressure roller 118 serving as a press-contact member in press-contact with the heating unit 126, and a fixing frame 121 serving as a holding member for holding the heating unit 126 and the pressure roller 118. A pressure plate 124 and a pressure spring 125 for pressing the heating unit 126 (pressure members) are provided in this fixing apparatus.
The heating unit 126 has a heater 115 serving as a heating element, a heater holder 117 serving as a heating element support member, and a fixing film 116 serving as a flexible sleeve that moves in contact with the heater 115. A fixing stay 119 is provided on one side of the heater holder 117, which is opposite to the side on which a heater element is held.
A flange 120 for regulating a longitudinal position of the fixing film 116 is provided at a longitudinal end portion of the fixing stay 119. The heating unit 126 is loosely and movably inserted into a guide groove 122 provided in the fixing frame 121.
The pressure roller 118 is axially supported by a bearing 123 mounted in the fixing frame 121. The pressure plate 124 acts as a lever and presses the heating unit 126 against the pressure roller 118 along the guide groove 122 provided in the fixing frame 121.
That is, one end 124a of the pressure plate 124 is passed through a hole 121b provided in a bent part 121a of the fixing frame 121 and serves as a fulcrum. The other end 124b serves as a force application point by arranging the compressed pressure spring 125 between the end 124b and the bent part 121c of the fixing frame 121.
An intermediate part of the pressure plate 124 act as a lever and presses a pressure portion 120c provided in the flange 120, so that the pressure portion 120c serves as a working point.
With the above pressurization configuration, a fixing nip N is constituted by the heater 115 and the pressure roller 118 via the fixing film 116.
The heating unit 126 is held by fitting the heater holder 117, which is located at the pressure roller 118 side with respect to the heating unit 126, and the flange 120 located at the other side, which is away from the pressure roller 128, into the guide groove 122 provided in the fixing frame 121.
That is, lower fitting portions 117a and 117b each for regulating a lower position of the heating unit 126 are provided at both ends of the heater holder 117. Upper fitting portions 120a and 120b each for regulating an upper position of the heating unit 126 are provided at both ends of the flange 120.
The heating unit 126 is held at the fixing frame 121 by inserting the lower fitting portions 117a and 117b and the upper fitting portions 120a and 120b into rims 122a and 122b respectively formed in both side edges of the guide groove 122.
In consideration of component dimension tolerances and component thermal expansion, the width of the guide groove 122 is set to be wider than each of the width between the lower fitting portions 117a and 117b and the width between the upper fitting portions 120a and 120b of the heating unit 126. In addition, a gap is provided between the guide groove 122 of the fixing frame 121 and each of the fitting portions of the heating unit 126.
The width of the guide groove 122 is equal to the span between the guide groove rim 122a at the downstream side in the recording material conveying direction and the guide groove rim 122b at the upstream side in the recording material conveying direction. Hereinafter, the “downstream side in the recording material conveying direction” and the “upstream side in the recording material conveying direction” are referred to simply as the “downstream side” and the “upstream side”, respectively.
The width between the upper fitting portions of the heating unit 126 is equal to the span between the downstream side opposite surface 120a and the upstream side opposite surface 120b of the flange portion 120 located at an upper side of the heating unit 126.
The width between the lower fitting portions of the heating unit 126 is equal to the span between the downstream side opposite surface 117a and the upstream side opposite surface 117b of the heater holder 117.
Next, a position of the heating unit in the guide groove 122 with a gap is described below. FIG. 15 illustrates external forces acting on the heating unit 126 except the fixing film 116. Symbols used in FIG. 15 represent the following elements.
P: force with which the pressure plate 124 presses the pressure portion 120c of the flange 120.
Nz: drag (normal force) applied from the pressure roller 118.
F: drag (normal force) received at the downstream side opposite surface 117a of the lower fitting portion of the heating unit 126 from the downstream side rim 122a of the guide groove 122 (if F<0, drag (normal force) received by the upstream side opposite surface 117b of the lower fitting portion of the heating unit 126 from the upstream side rim 122b of the guide groove 122).
G: drag (normal force) received at the downstream side opposite surface 120a of the upper fitting portion of the heating unit 126 from the downstream side rim 122a of the guide groove 122 (if G<0 drag (normal force) received by the upstream side opposite surface 117b of the lower fitting portion of the heating unit 126 from the upstream side rim 122b of the guide groove 122).
μ: friction coefficient between the fixing film 116 and the heater 115.
a: distance from a fixing nip surface to each of the lower fitting portions 117a and 117b of the heating unit 126.
b: distance from the fixing nip surface to each of the upper fitting portions 120a and 120b of the heating unit 126.
A balance equation of force acting in a z-direction (direction parallel to the guide groove), a balance equation of force acting in a y-direction (direction perpendicular to the guide groove), and a balance equation of rotation moment around a point (fixing nip center) O are as follows.Force acting in the z-direction: P=Nz Force acting in the y-direction: F+G=μNz Rotation moment around the point O: aF+bG=0
According to the above three equations, when the pressure roller rotates in a direction in which a recording material is conveyed, the drags (normal forces) F and G are given as follows.F=μbP/(a+b)>0G=−μaP/(a+b)<0
That is, a lower part of the heating unit 126 abuts against the downstream side rim of the guide groove 122, while an upper part of the heating unit 126 abuts against the upstream side rim of the guide groove 122. Thus, the heating unit 126 is tilted in the guide groove 122.
In this case, the position of the heating unit 126 is affected by the span between the fitting portions of the heating unit, the span of the guide groove in various dimensions thereof.
In a case where dimensions of many types of components of the heating unit 126 affect the position thereof, the position of the heating unit 126 can be changed as much as the tolerances of the dimensions. When the position of the heating unit 126 is changed, the relative position of the heater 115 with respect to the fixing nip N may be changed.
Consequently, sometimes, a heat distribution in the fixing nip N changes, so that a fixing failure and an image defect, such as a cold offset or a hot offset, may occur, as discussed in Japanese Patent Application Laid-Open No. 2006-171630.